A promoter-linked coupling region required for stimulation of alpha-fetoprotein transcription by distant enhancers

Loss of Nkx2.8 deregulates progenitor cells in the large airways and leads to dysplasia

Nkx2.8, a homeodomain transcription factor, has been characterized in liver cancer and in the developing central nervous system. We now show that this factor is also expressed in the lung, where it localizes in adults to a discrete population of tracheobronchial basal cells. To target the mouse gene, the first exon was replaced by a LacZ marker gene joined to the intact 5'-untranslated region. Marker expression was observed throughout the lower respiratory tract, beginning on E11 in a few cells of the distal lung buds. The region of expression then spread upward. By neonatal day 1, expression was greatest in the large airways and the Nkx2.8-/- mice exhibited generalized tracheobronchial hyperplasia. Bromodeoxyuridine (BrdUrd) labeling studies showed that a higher rate of bronchial cell proliferation persisted at 6 to 8 months. In adults, Nkx2.8 marker expression decreased with progressive differentiation into ciliated and secretory cells. The cell localizations and patterns of coexpression with BrdUrd and differentiation markers suggest a progenitor relationship: the cells that most strongly express Nkx2.8 seem to function as tracheobronchial stem cells. Moreover, Nkx2.8 acts to limit the number of these progenitor cells because the marker-expressing population was greatly expanded in Nkx2.8-/- mice. Increased proliferation and an altered progenitor relationship caused progressive bronchial pathology, which manifested as widespread dysplasia in the large airways of 1-year-old Nkx2.8-/- mice.

The role of proximal-enhancer elements in the glucocorticoid regulation of carbamoylphosphate synthetase gene transcription from the upstream response unit

As part of the urea cycle, carbamoylphosphate synthetase (CPS) converts toxic ammonia resulting from amino-acid catabolism into urea. Liver-specific and glucocorticoid-dependent expression of the gene involves a distal enhancer, a promoter-proximal enhancer, and the minimal promoter itself. When challenged with glucocorticoids, the glucocorticoid-responsive unit (GRU) in the distal enhancer of the carbamoylphosphate-synthetase gene can only activate gene expression if, in addition to the minimal promoter, the proximal enhancer is present. Here, we identify and characterise two elements in the proximal CPS enhancer that are involved in glucocorticoid-dependent gene activation mediated by the GRU. A purine-rich stretch forming a so-called GAGA-box and a glucocorticoid-response element (GRE) are both crucial for the efficacy of the GRU and appear to constitute a promoter-proximal response unit that activates the promoter. The glucocorticoid response of the CPS gene is, therefore, dependent on the combined action of a distal and a promoter-proximal response unit.

Afr2 regulation occurs cell-autonomously in vitro but is not conferred on episomal DNA in transient assays

Oncofetal antigens such as alpha-fetoprotein (AFP) are expressed in regenerating liver. The level of AFP gene expression during liver regeneration is regulated by the unlinked, autosomal gene, Alpha-fetoprotein regulator 2 (Afr2). C3H/HeJ (Afr2A/A) mice express 10-fold higher levels of AFP than C57BL/6J (Afr2B/B) mice. Here we show that primary hepatocytes isolated from C3H/HeJ and C57BL/6J mice exhibit differential expression of the endogenous AFP gene, which was attributed to the Afr2 gene locus and indicative of a cell-autonomous mechanism. We show that the Afr2-Response Element (ARE), between 1010 and 838 base pairs upstream of the AFP transcriptional start site, did not modulate reporter gene expression in transfection assays of Hep G2, Hep 3B, Hepa 1.6, and HeLa cell lines. Reporter gene expression in transiently transfected primary hepatocytes was also ARE-independent. Finally, gene expression from reporter constructs delivered by hydrodynamics-based transfection to the livers of C3H/HeJ and C57BL/6J mice after CCl4-induced liver regeneration was ARE-independent. In conclusion, ARE-dependent transcription was not found in transient assays performed in three different systems, two of which retained regulation of the endogenous AFP gene, suggesting that the ARE may not function as a simple transcription factor recognition site.

Regulation of alpha-fetoprotein expression by Nkx2.8

The alpha-fetoprotein (AFP) gene is an important model of developmental gene silencing and neoplastic gene reactivation. Nkx2.8 is a divergent homeodomain factor originally cloned through its binding to the promoter-coupling element (PCE), a regulatory region upstream of the AFP promoter that mediates stimulation by distant enhancers. Nkx2.8 is the only developmentally regulated factor that has been associated with AFP gene expression. Fetoprotein transcription factor, an orphan nuclear receptor, has also been shown to bind the PCE but is not developmentally regulated. The binding specificities of both families of transcription factor were determined, and overlapping sites for each were defined in the PCE. After modification of nuclear extract and gel shift analysis procedures, Nkx2.8 was identified in six AFP-positive cell lines. Transient-transfection analysis did not show transcriptional stimulation by Nkx2.8 or other active NK2 factors, which only interfered with gene expression. However, two sets of analysis demonstrated the relationship of Nkx2.8 to AFP expression: chromatin immunoprecipitation demonstrated that Nkx2.8 bound to the active AFP promoter, and antisense inhibition of Nkx2.8 mRNA translation selectively reduced expression of both the endogenous human AFP gene and transfected reporters containing the rat AFP promoter.

The mouse alpha-fetoprotein promoter is repressed in HepG2 hepatoma cells by hepatocyte nuclear factor-3 (FOXA)

The mouse alpha-fetoprotein gene is expressed at high levels in the fetal liver and is transcriptionally silenced at birth. The repression is governed, at least in part, by the 250 base pair (bp) AFP promoter. We show here that the AFP promoter is dramatically repressed by HNF3 in HepG2 hepatoma cells. This repression is governed by the region between -205 and -150. Furthermore, this fragment can confer HNF3-mediated repression on a heterologous promoter. The repression is abolished by a mutation that is centered at -165. EMSA analyses using in vivo and in vitro synthesized proteins indicate that HNF3 proteins do not bind DNA from the -205 to -150 region. We propose that HNF3 represses AFP promoter activity through indirect mechanisms that modulate the binding or activity of a liver-enriched factor that interacts with the -165 region of the AFP promoter.

Distant enhancers stimulate the albumin promoter through complex proximal binding sites

The albumin-alpha-fetoprotein locus epitomizes the main features of transcriptional regulation of fetal and adult hepatocyte-specific genes: developmentally regulated promoters and strong distant enhancers. Full enhancer activity required only a proximal albumin-promoter region containing the TATA box, hepatic nuclear factor 1 (HNF1), and nuclear factor Y (NF-Y) sites. Deletion of the HNF1 site abrogated enhancer and promoter activity, whereas methylation of the site reduced all activity by about 3-fold. Deletion of the NF-Y site attenuated activity by about half, but much of the activity could be replaced by juxtaposition of an upstream region (designated distal element IV). Gel shift and competition analysis demonstrated that binding of architectural factors overlapped NF-Y binding. Moreover, a mutation that eliminated NF-Y binding but only minimally perturbed the surrounding region did not affect enhancer function. In plasmids with a second promoter, the enhancers simultaneously stimulated both albumin and alpha-fetoprotein promoters with minimal competition, but surprisingly some mutations in the albumin promoter attenuated expression from both promoters, whereas another uncoupled their expression. With single promoters, the function of the proximal promoter region was controlled by three parameters in the following hierarchy: HNF1 binding > local architecture > NF-Y binding, but integrated two-promoter function had a much greater dependence on NF-Y.

Hepatitis B viral transactivator HBx alleviates p53-mediated repression of alpha-fetoprotein gene expression

Chronic infection with hepatitis B virus (HBV) is associated with development of hepatocellular carcinoma (HCC). The exact mechanism by which chronic infection with HBV contributes to onset of HCC is unknown. However, previous studies have implicated the HBV transactivator protein, HBx, in progression of HCC through its ability to bind the human tumor suppressor protein, p53. In this study, we have examined the ability of HBx to modify p53 regulation of the HCC tumor marker gene, alpha-fetoprotein (AFP). By utilizing in vitro chromatin assembly of DNA templates prior to transcription analysis, we have demonstrated that HBx functionally disrupts p53-mediated repression of AFP transcription through protein-protein interaction. HBx modification of p53 gene regulation is both tissue-specific and dependent upon the p53 binding element. Our data suggest that the mechanism by which HBx alleviates p53 repression of AFP transcription is through an association with DNA-bound p53, resulting in a loss of p53 interaction with liver-specific transcriptional co-repressors.

The hepatitis B virus core promoter is strongly activated by the liver nuclear receptor fetoprotein transcription factor or by ectopically expressed steroidogenic factor 1

Orphan nuclear receptor fetoprotein transcription factor (FTF) was previously identified as a specific regulator of the alpha(1)-fetoprotein gene during early liver development and in response to hormonal signals (L. Galarneau, J.-F. Paré, D. Allard, D. Hamel, L. Lévesque, J. D. Tugwood, S. Green, and L. Bélanger, Mol. Cell. Biol. 16:3853-3865, 1996). Here we report a functional analysis of FTF interactions with the hepatitis B virus (HBV) nucleocapsid promoter. DNA-protein-binding assays show that the HBV core promoter contains two high-affinity FTF-binding sites and a third, lower-affinity site shared with other receptors. Transfections in HepG2, Hep3B, and PLC/PRF/5 hepatoma cells using chloramphenicol acetyltransferase reporter genes with the nucleocapsid promoter linked or not linked to enhancer I indicate that FTF is a potent activator of the HBV core promoter, more efficient than HNF4alpha, HNF3alpha, HNF3beta, or C/EBPalpha. Steroidogenic factor 1, a close FTF homolog which binds to the same DNA motif and is expressed ectopically in HepG2 cells, seems to be an even stronger inducer than FTF. Point mutations of the FTF-binding sites indicate direct FTF activatory effects on the core promoter and the use of both high-affinity sites for productive interaction between the core promoter and enhancer I. Coexpression assays further indicate that FTF and HNF4alpha are the most efficient partners for coactivation of the pregenomic core promoter, which may largely account for the hepatic tropism and the early amplification of HBV infection. Carboxy terminus-truncated FTF behaves as a dominant negative mutant to compete all three FTF sites and strongly deactivate core promoter interactions with enhancer I; this suggests possible new ways to interfere with HBV infection.

Retinoic acid mediates down-regulation of the alpha-fetoprotein gene through decreased expression of hepatocyte nuclear factors

alpha-Fetoprotein (AFP), a protein highly induced during fetal liver development, is down-regulated by retinoids in the human hepatoma cell line Hep3B, in contrast to up-regulation observed in other cell types. Previously, we have documented that such up-regulation involves direct effects through cis-retinoid X receptor-binding sites in the AFP enhancer. In this report, we show a distinctive effect of all-trans-retinoic acid (RA) in Hep3B cells. RA caused a marked decrease in AFP transcripts. Deletion analysis of the upstream regulatory region of the AFP gene revealed that cis-acting sites required for down-regulation resided near the promoter. Gel mobility shift assays for factors binding to key elements in the AFP promoter region demonstrated that hepatocyte nuclear factor (HNF) 1 binding was diminished in nuclear extracts from RA-treated cells. In addition, HNF4, which is not known to bind to the AFP promoter but does regulate HNF1, was also diminished. The levels of HNF1 and HNF4 mRNA were also decreased following RA treatment. AFP promoter-chloramphenicol acetyltransferase transient transfection assays demonstrated that the level of HNF1 had a direct impact on basal transcription as well as RA-mediated down-regulation of the AFP gene, and that co-transfection of HNF1 and HNF4, but not transfection of either factor alone, reversed the RA-mediated inhibition. Taken together these data point to an interaction among the RA, HNF1, and HNF4 signals, which is reflected in decreased expression of AFP.

alpha-Fetoprotein gene sequences mediating Afr2 regulation during liver regeneration

alpha-fetoprotein (AFP) gene expression occurs in the yolk sac, in the fetal liver and gut, and in the adult liver during regeneration and tumorigenesis. Two unlinked genes determine the level of AFP gene expression in adult mice: Afr1 regulates the basal level of expression in the normal adult liver, and Afr2 regulates the increase in expression during liver regeneration. It has been shown that AFP-derived transgenes, including the sequences between -1,010 and -838 bp and between -118 bp and the transcriptional start site were induced appropriately during liver regeneration and were Afr2-regulated. To assess the role of the distal sequence in gene expression during liver regeneration, a new transgene with 7.6 kilobases of 5'-flanking sequence deleted between -1,010 and -838 bp was designed. We show that this transgene was subject to characteristic AFP tissue-specific and developmental regulation, in that it was highly expressed in the yolk sac and the fetal liver and gut but not in normal adult tissues. Expression was induced in response to liver regeneration as observed for the endogenous gene. The genetic regulation of the basal level of AFP gene expression in adult liver by the Afr1 gene was undisturbed. However, transgene expression was not regulated by Afr2 during liver regeneration. Our data suggest that Afr2 regulation of AFP gene expression during liver regeneration requires the sequence between -1,010 and -838 bp and is independent of other regulatory mechanisms.

A novel nk-2-related transcription factor associated with human fetal liver and hepatocellular carcinoma

A novel cDNA was partially isolated from a HepG2 cell expression library by screening with the promoter-linked coupling element (PCE), a site from the alpha-fetoprotein (AFP) gene promoter. The remainder of the cDNA was cloned from fetal liver RNA using random amplification of cDNA ends. The cDNA encodes a 239-amino acid peptide with domains closely related to the Drosophila factor nk-2. The new factor is the eighth vertebrate factor related to nk-2, hence nkx-2.8. Northern blot and reverse transcriptase polymerase chain reaction analysis demonstrated mRNA in HepG2, two other AFP-expressing human cell lines, and human fetal liver. Transcripts were not detected in adult liver. Cell-free translation produced DNA binding activity that gel shifted a PCE oligonucleotide. Cotransfection of nkx-2.8 expression and PCE reporter plasmids into HeLa cells demonstrated transcriptional activation; NH2-terminal deletion eliminated this activity. Cotransfection into AFP-producing hepatocytic cells repressed AFP reporter expression, suggesting that endogenous activity was already present in these cells. In contrast, cotransfection into an AFP-negative hepatocytic line produced moderate activation of the AFP gene. The cardiac developmental factor nkx-2.5 could substitute for nkx-2.8 in all transfection assays, whereas another related factor, thyroid transcription factor 1, showed a more limited range of substitution. Although the studies have yet to establish definitively that nkx-2.8 is the AFP gene regulator PCF, the two factors share a common DNA binding site, gel shift behavior, migration on SDS-acrylamide gels, and cellular distribution. Moreover, the nk-2-related genes are developmental regulators, and nkx-2.8 is the first such factor associated with liver development.

RXR-mediated regulation of the alpha-fetoprotein gene through an upstream element

Retinoic acid (RA) is known to have potent effects on development and differentiation. RA exerts its effects on transcription through two distinct classes of nuclear receptors, the retinoic acid receptor (RAR) and the retinoid X receptor (RXR), that bind to specific RA-responsive elements (RARE) in target genes. alpha-Fetoprotein (AFP), a hepatocyte differentiation, maturation, and carcinogenesis marker, is transcriptionally upregulated by RA in McA-RH8994 hepatoma cells. Using deletion mapping analysis, we have identified a RARE-like sequence that is located between -2406 and -2378 of the transcription initiation site of the rat AFP gene. Sequence analysis demonstrated that this cis-acting element consists of three direct repeats and one inverted repeat of a GGGTCA-like half-site. The putative RARE can specifically bind to both RXR homodimers and RAR/RXR heterodimers as determined by gel mobility shift assays. A DR1 direct repeat was more efficient than a DR5 direct repeat oligonucleotide in competition for binding of the putative RARE to RXR and RAR/RXR. A mutagenesis study indicated that to have a full-strength induction, all the repeats were required. To further analyze the function of this element in vivo, a reporter gene construct of the putative RARE combined with the thymidine kinase promoter was cotransfected with RAR and RXR expression plasmids in CV1 cells. CAT assays demonstrated that overexpression of RXRalpha conferred the best RA response, consistent with our previous observation that 9-cis-RA is more potent than all-trans-RA for inducing the expression of the AFP gene. In addition, the RXR selective ligand LG100153 alone can stimulate the expression of the AFP gene. Our data suggest that an RXR-mediated pathway exists for modulation of AFP gene expression through a specific element.

In vitro and in vivo hepatoma cell-specific expression of a gene transferred with an adenoviral vector

Recombinant adenoviruses are widely used for the transfer of foreign genes into various mammalian cells. However, the utilization of these vectors for cancer gene therapy requires the specific and efficient expression of the transferred gene in tumor cells. To obtain targeted expression in hepatoma cells, we constructed recombinant adenoviral vectors containing transcriptional elements from either the rat alpha-fetoprotein (AFP) or the human insulin-like growth factor II (IGFII) genes driving expression of the nuclear beta-galactosidase gene (nls lacZ). In vitro infection revealed that the AFP but not the IGFII transcriptional regulatory sequence controlled nls lacZ expression specifically in hepatoma cells. The same specificity was obtained in vivo in subcutaneous human hepatic tumors generated by engraftment of Huh7 hepatoma cells in nude mice as well as in primary liver tumors developed in rats and mice. No marker gene expression was detectable after AFP-nls lacZ gene transfer to normal rat liver parenchyma despite evidence for the presence of DNA encoding the nls lacZ gene. However, in vivo experiments with primary liver tumors in rats and mice also revealed that primary hepatoma cells were poorly infected by adenoviral vectors. Peritumoral and normal tissues were infected efficiently by adenoviral vectors. We conclude that hepatoma cell-specific expression of a transgene can be achieved with AFP regulatory sequences but that adenoviral vectors may not be the preferable vector for transferring genes in vivo in primary liver tumors.

Sequence requirements for Afr-2 regulation of alpha-fetoprotein gene expression during liver regeneration

Alpha-fetoprotein (AFP) gene expression occurs in the yolk sac, fetal liver and gut, and in the adult liver during regeneration and tumorigenesis. Polymorphism at a single genetic locus, Afr-2 (formerly known as Rif) between inbred mouse strains C3H/He and C57B1/6, results in different levels of AFP expression during liver regeneration. We examined AFP, histone H3, and albumin gene expression during liver regeneration and found that the strain-specific variance in AFP gene expression could not be attributed to a difference in the numbers of dividing cells. Experiments with transgenic mice revealed sequences required for Afr-2 regulation included 172 bp between -1010 and -838 bp and 118 bp immediately upstream of the AFP transcriptional start site-the same regions required for induction during liver regeneration. This suggests that the Afr-2 phenotype may stem from an allelic difference in a gene regulating gene expression during liver regeneration.

Individual mouse alpha-fetoprotein enhancer elements exhibit different patterns of tissue-specific and hepatic position-dependent activities

Transcription of the mouse alpha-fetoprotein (AFP) gene, which is expressed at high levels in the visceral endoderm of the yolk sac and fetal liver and at low levels in the fetal gut, is regulated by three distinct upstream enhancer regions. To investigate the activities of these regions, each enhancer was individually linked to a heterologous human beta-globin promoter fused to the mouse class I H-2Dd structural gene. When tested in transgenic mice, the beta-globin promoter alone has minimal activity. We find that all three enhancers activate the beta-globin promoter in an AFP-like pattern; i.e., activity is detected in the yolk sac, fetal liver, and fetal gut. The enhancers remain active in the livers and guts of adult mice, consistent with previous studies showing that postnatal AFP repression is due not to the loss of enhancer activity but to a dominant repressor region. Enhancer III also functions in the brain. In addition, these studies reveal that the three enhancers exhibit different position-dependent activities in the adult liver. Enhancers I and II are most active in hepatocytes surrounding the central vein, with a gradual decrease in activity along the hepatic plates toward the portal triad. Enhancer III is active exclusively in hepatocytes surrounding the central vein. These data represent the first examples of individual control elements exhibiting positionally regulated activity in adult liver.

Enhancer sharing in a plasmid model containing the alpha-fetoprotein and albumin promoters

It has been proposed that the alpha-fetoprotein (AFP) enhancers also regulate the adjacent albumin gene, since the -10 kbp albumin enhancer is inactive in a number of cell lines that express albumin. In transfection experiments, the AFP enhancers strongly stimulate the albumin promoter in cells that silence the AFP promoter. These observations led us to develop a plasmid model of AFP-albumin gene switching, in which the albumin and AFP promoters would compete for the three AFP enhancers. However, when AFPCAT + ALBgal genes were combined with the AFP enhancers in one plasmid, both genes were driven at full activity. There was no change in the relative promoter expression over a wide range of transfected DNA concentrations, demonstrating that relative promoter activity was independent of DNA concentration and of promoter concentration, and that neither promoter was limiting the expression of the other. In contrast, a control plasmid containing two albumin promoters showed mutual inhibition, indicating the expected promoter competition. The albumin and AFP promoters noncompetitively shared the three enhancers on this plasmid, resulting in high levels of transcription from both promoters.

A novel hepatocytic transcription factor that binds the alpha-fetoprotein promoter-linked coupling element

We recently characterized a promoter-linked coupling element (PCE) in the rat alpha-fetoprotein (AFP) gene required for strong transcriptional stimulation by distant enhancers (P. Wen, N. Crawford, and J. Locker, Nucleic Acids Res. 21:1911-1918, 1993). In this study, oligonucleotide gel retardation and competition experiments defined the PCE as a 12-bp binding site, TGTCCTTGAACA, an imperfect inverted repeat from -166 to -155 near the AFP promoter. A factor that bound this site (PCF) was abundant in HepG2 nuclear extracts and detectable in extracts from several other AFP-producing hepatocarcinoma cell lines and fetal liver. Hepatocytic cell lines that did not express AFP, nonhepatocytic cell lines, adult liver, and fetal brain did not show the factor. Experiments excluded the possibility that PCF activity was due to binding of glucocorticoid receptor or an AP1-like factor that bound overlapping sites. Competition experiments with several mutant oligonucleotides determined that the optimum PCF binding site was TGTCCTTGAAC(A/T). Mutations decreased binding or totally abolished binding activity. In expression plasmids, PCE mutations strongly reduced gene expression. UV cross-linking to a PCE probe identified peptide bands near 34 kDa. PCF was purified by heparin-Sepharose chromatography followed by affinity binding to oligomerized PCE DNA. The product resolved as a complex of three peptides (PCF alpha 1, PCF alpha 2, and PCF beta, 32 to 34 kDa) on sodium dodecyl sulfate-acrylamide gels. The peptide sizes and gel patterns are unlike those of any of the well-described hepatic transcription factors, and the binding site has not been previously reported. PCF thus appears to be a novel transcription factor.

A novel hepatocytic transcription factor that binds the alpha-fetoprotein promoter-linked coupling element

We recently characterized a promoter-linked coupling element (PCE) in the rat alpha-fetoprotein (AFP) gene required for strong transcriptional stimulation by distant enhancers (P. Wen, N. Crawford, and J. Locker, Nucleic Acids Res. 21:1911-1918, 1993). In this study, oligonucleotide gel retardation and competition experiments defined the PCE as a 12-bp binding site, TGTCCTTGAACA, an imperfect inverted repeat from -166 to -155 near the AFP promoter. A factor that bound this site (PCF) was abundant in HepG2 nuclear extracts and detectable in extracts from several other AFP-producing hepatocarcinoma cell lines and fetal liver. Hepatocytic cell lines that did not express AFP, nonhepatocytic cell lines, adult liver, and fetal brain did not show the factor. Experiments excluded the possibility that PCF activity was due to binding of glucocorticoid receptor or an AP1-like factor that bound overlapping sites. Competition experiments with several mutant oligonucleotides determined that the optimum PCF binding site was TGTCCTTGAAC(A/T). Mutations decreased binding or totally abolished binding activity. In expression plasmids, PCE mutations strongly reduced gene expression. UV cross-linking to a PCE probe identified peptide bands near 34 kDa. PCF was purified by heparin-Sepharose chromatography followed by affinity binding to oligomerized PCE DNA. The product resolved as a complex of three peptides (PCF alpha 1, PCF alpha 2, and PCF beta, 32 to 34 kDa) on sodium dodecyl sulfate-acrylamide gels. The peptide sizes and gel patterns are unlike those of any of the well-described hepatic transcription factors, and the binding site has not been previously reported. PCF thus appears to be a novel transcription factor.

Modulation of transcriptional activation of the proliferating cell nuclear antigen promoter by the adenovirus E1A 243-residue oncoprotein depends on proximal activators

Previous analyses defined a proliferating cell nuclear antigen (PCNA) E1A-responsive element (PERE) in the PCNA promoter that is essential for transactivation by the 243-residue product of the adenovirus type 2 E1A 12S mRNA (E1A 243R). In this report, we show that the PERE activates a heterologous basal promoter and confers susceptibility to transactivation by E1A 243R, indicating that the PERE is both necessary and sufficient for the response of the PCNA promoter to this oncoprotein. Insertion of linker sequences between the PERE and the site of transcription initiation in the PCNA promoter severely impairs the promoter's response to E1A 243R transactivation. GAL4 sites can replace the function of the PERE in the E1A 243R response of the PCNA basal promoter if transcriptional activators of suitable strength are supplied as GAL4 fusion proteins. Weak transcriptional activators render the PCNA basal promoter subject to transactivation by E1A 243R but do not endow the adenovirus E1B basal promoter with a similar response. Strong transcriptional activators do not support transactivation by E1A 243R, however; instead, E1A reduces the ability of the strong activators to activate both the PCNA and E1B basal promoters. Although other mechanistic differences might determine the response, the data imply a relationship between the activation strength of promoter-proximal effectors and the response of the PCNA basal promoter to E1A 243R. These experiments indicate that the PERE can function autonomously in mediating transactivation by E1A 243R and that the PCNA basal promoter is configured in a manner that permits modulation by E1A 243R of transcriptional activation by promoter-proximal effectors.

Modulation of transcriptional activation of the proliferating cell nuclear antigen promoter by the adenovirus E1A 243-residue oncoprotein depends on proximal activators

Previous analyses defined a proliferating cell nuclear antigen (PCNA) E1A-responsive element (PERE) in the PCNA promoter that is essential for transactivation by the 243-residue product of the adenovirus type 2 E1A 12S mRNA (E1A 243R). In this report, we show that the PERE activates a heterologous basal promoter and confers susceptibility to transactivation by E1A 243R, indicating that the PERE is both necessary and sufficient for the response of the PCNA promoter to this oncoprotein. Insertion of linker sequences between the PERE and the site of transcription initiation in the PCNA promoter severely impairs the promoter's response to E1A 243R transactivation. GAL4 sites can replace the function of the PERE in the E1A 243R response of the PCNA basal promoter if transcriptional activators of suitable strength are supplied as GAL4 fusion proteins. Weak transcriptional activators render the PCNA basal promoter subject to transactivation by E1A 243R but do not endow the adenovirus E1B basal promoter with a similar response. Strong transcriptional activators do not support transactivation by E1A 243R, however; instead, E1A reduces the ability of the strong activators to activate both the PCNA and E1B basal promoters. Although other mechanistic differences might determine the response, the data imply a relationship between the activation strength of promoter-proximal effectors and the response of the PCNA basal promoter to E1A 243R. These experiments indicate that the PERE can function autonomously in mediating transactivation by E1A 243R and that the PCNA basal promoter is configured in a manner that permits modulation by E1A 243R of transcriptional activation by promoter-proximal effectors.